The present invention relates to an inverter with a mains system side and load-side freewheeling pulse converter, and more particularly to an inverter which can operate at higher pulse frequency and with a higher DC intermediate voltage than conventional inverters
An inverter with freewheeling pulse converters on the mains system side and load-side, which are electrically connected on the DC side via a DC intermediate circuit, is commercially available. Inverters of this type are known, for example, from the Siemens catalog DA65.10.2000, entitled “Simovert Masterdrives Vector Control”. In this Siemens catalog, the diagram “Reduction Curves for the Motor-Side Converter” show that under standard operating conditions the pulse frequency fpL is, for example, 6 kHz which can be increased to a maximum pulse frequency of fpL max=16 kHz. However, an increase in the pulse frequency fpL to 16 kHz reduces the current rating by 50%, which also reduces by 50% the power available from the converter.
The conventional inverter described above can keep feedback to the power mains system to a minimum. A power factor of one is set for the mains system side freewheeling converter, so that only active power is supplied by the power mains. A supply controlled in this fashion is also referred to as an Active Front End (AFE).
The conventional inverter has switchable semiconductor switches which are implemented as insulated gate bipolar transistors (IGBT) and made of silicon (Si). The electrical losses in Si-components increase superlinearly with the operating voltage, so that the maximum blocking voltage has to be optimized. When operating from a conventional line voltage, conventional inverters have typically a blocking voltage of 1200 V. However, switching losses limit the switching frequency. The switching frequency is typically between 3 and 8 kHz. This switching frequency makes it difficult to optimally dimension the line filter and/or the output filter of the inverter. n addition, the current for a predetermined power is determined by the maximum available voltage, which in turn determines the size of the motor cables.
It would be desirable and advantageous to provide an improved inverter with a higher voltage rating, in particular in the intermediate DC circuit.